1. Field of the Invention
The invention generally relates to methods and apparatus for performing digital magnetic recording and readback. More particularly, the invention relates to methods and apparatus for performing high density isotropic/perpendicular recording of digital information on magnetic media where the distribution of magnetization on the media correlates well with the fringing field of the readback apparatus.
2. Description of the Related Art
Techniques for the storage of digital information in a magnetizable medium are well known. These techniques are generally based on interaction between a magnetic storage medium and a magnetic head (transducer), in relative motion with respect to one another. The magnetic head is typically comprised of a magnetic core wound with a coil, where the core includes a head gap. The gap field, with its surrounding leakage flux ("fringing field") extends into the magnetic recording surface and leaves a magnetic remanent on the storage media in write mode.
The head provides an induced voltage on readback, reflecting the rate of change of magnetization recorded on the magnetic "track." Thus, the quality of the readback output of a magnetic recording is directly related to and the correlation between the fringing field of the readback head and the derivative of the remanent media magnetization.
Known digital magnetic recording systems which attempt to maximize their data storage capacity typically suffer a degradation in readback performance. A factor in achieving maximum data storage capacity is the information density per unit area of storage surface. The storage density per unit surface area is the product of the storage density per unit track length times the track density per unit distance normal to the direction of relative motion. These two components of storage density are interrelated; an increase in track density will cause a reduction in available readback flux. This has an adverse affect on readback performance, particularly where the distribution of magnetization on the media does not correlate well with the fringing field of the readback apparatus.
By way of background, it should be understood that conventional digital magnetic recording systems use one or more of three primary modes of recording. These can be defined in terms of the direction of surface magnetization relative to the direction of track motion. These modes are: (1) longitudinal or horizontal recording, (2) perpendicular or vertical recording, and (3) transverse recording.
In longitudinal recording, the principal direction of magnetization is in the plane of the surface and parallel to the direction of surface motion. In vertical recording, the principal orientation of the magnetization is normal to the plane of the surface. In transverse recording, the storage medium is magnetized in the plane of its surface but normal to the direction of motion between the head and surface. These recording modes are well known in the prior art.
In addition to the recording modes and by way of further background, it should be understood that conventional systems create the remanent media magnetization itself by one of two methods. The first method requires switching the polarity of the continuous current in the winding of the write head. The second method requires the application of short current pulses in the head winding.
According to the first, most frequently used method, the value of 0's of encoded bits is represented by the constant state of media magnetization, while the value of 1's of encoded bits is represented by the change of the state of media magnetization. This change is accomplished by the change in the polarity of the continuous write current in the head winding. As moving media passes the head it is magnetized by the field created by the constant current in the winding to the one definite state. At the moments of the changes in current polarity, corresponding changes in the polarity of remanent media magnetization occur at the trailing edge of the head. These changes are formed in the area where the intensity of the head field decreases to the value of the media coercivity. The "recording zone" for this method is outside of the area of the most intensive gap field, i.e. where the field is much greater then media coercivity.
According to the second conventional recording method, which may be used to record on media that is previously demagnetized, short current pulses of different polarity in the head winding create corresponding intervals of saturated magnetization, of different polarity, on the previously demagnetized media. One polarity corresponds to 1's; the other polarity corresponds to the 0's of the encoded bits. In this case, changes in the media magnetization occurs at both the trailing and leading edges of the head, outside of the gap. Therefore, the "recording zones" again do not coincide with the area of most intensive head field in the gap. In practical situations, "imprints" of each current pulse are much wider than the gap size.
According to a variant of this second method, where previously saturated media is used, current pulses are always of the same polarity. The pulses create media intervals where magnetization polarity is reversed relative to the prerecorded state. These revisions represent the 1's of the encoded bits; 0's being represented by the absence of the changes in the media magnetization. Otherwise, the method is similar to the aforesaid second conventional recording method.
A common disadvantage of both of the methods described hereinabove, is the under utilization of the write field of the head during recording. Due to highly assymmetrical and skewed nature of the magnetic field outside the gap, changes in the media magnetization written "by the edges of the head" on different layers of media are out of correct phase in relation to each other. Additionally, due to the insufficient gradient of the head field outside of the gap, written changes in magnetization are blurred.
As a result, written patterns of the recorded bits, recorded via the prior art methods, are blurred and do not correlate well with the whole fringing field of the readback transducer. Therefore, the resolution and signal to noise ratio in the readback signal suffer.
U.S. Pat. No. 3,564,558, issued Aug. 26, 1968 to Tolman et al, entitled "High-Density Magnetic Recording Scheme", dealing with surface recording on extremely thin magnetic media using a combination of transverse and longitudinal magnetic write fields, typifies the prior art approach to recording. Tolman utilizes the trailing edge of the write field for recording, in other words, recording occurs in the area where the write field reduces itself to the value of media coercivity. Hence, once again, the distribution of magnetization on the media will not be well correlated with the fringing field of the readback transducer.